mus: Make --use-mus-in-renderer work again.

. It is necessary to send the --use-mus-in-renderer flag to the renderer.
. Allow content_browser to connect to the renderer and get a handle to
  a RenderWidgetWindowTreeClientFactory.

BUG=none

Review-Url: https://codereview.chromium.org/2037533002
Cr-Commit-Position: refs/heads/master@{#397759}

content/browser/renderer_host/render_process_host_impl.cc

@@ -1565,6 +1565,9 @@ void RenderProcessHostImpl::PropagateBrowserCommandLineToRenderer(
 #endif
 #if defined(OS_CHROMEOS)
     switches::kDisableVaapiAcceleratedVideoEncode,
+#endif
+#if defined(MOJO_SHELL_CLIENT)
+    switches::kUseMusInRenderer,
 #endif
   };
   renderer_cmd->CopySwitchesFrom(browser_cmd, kSwitchNames,

content/public/app/mojo/content_browser_manifest.json

@@ -6,6 +6,9 @@
   "capabilities": {
     "required": {
       "*": { "classes": [ "app" ] },
+      "exe:content_renderer": {
+        "interfaces": [ "content::mojom::RenderWidgetWindowTreeClientFactory" ]
+      },
       "mojo:shell": {
         "classes": [
           "shell:client_process",